Reinstate Search Functionality

Summary of changes:
* Reinstates search bar
* Updates API end points to return content data
* Cleans up autocomplete.js
* Makes search interact properly with sidebar navigation
* Enables search by keyword

This is a minimum viable implementation for WCH, needs refactoring and more robust implementation for 0.13.

@jamalex @dylanjbarth please review.
This is legacy code -- you can just request the flattened topic tree from the server directly.